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arXiv:2304.14548 · Physical Review A

Optical cycling in polyatomic molecules with complex hyperfine structure

Yi Zeng, Arian Jadbabaie, Ashay N. Patel, Phelan Yu, Timothy C. Steimle, Nicholas R. Hutzler

Published Jul 17, 2023• 12 citations
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Abstract

We have developed and demonstrated a scheme to achieve rotationally closed photon cycling in polyatomic molecules with complex hyperfine structure and sensitivity to hadronic symmetry violation, specifically, $^{171}\mathrm{YbOH}$ and $^{173}\mathrm{YbOH}$. We calculate branching ratios for spontaneous decay into the different hyperfine levels and identify repumping schemes. We demonstrate our scheme by cycling photons in a molecular beam and verify that we have achieved rotationally closed cycling by measuring optical pumping into unaddressed vibrational states. Our work makes progress along the path toward utilizing photon cycling for state preparation, readout, and laser cooling in precision measurements of polyatomic molecules with complex hyperfine structure.

DOI
10.1103/physreva.108.012813

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We have developed and demonstrated a scheme to achieve rotationally closed photon cycling in polyatomic molecules with complex hyperfine structure and sensitivity to hadronic symmetry violation, specifically, 171YbOH^{171}\mathrm{YbOH}171YbOH and 173YbOH^{173}\mathrm{YbOH}173YbOH. We calculate branching ratios for spontaneous decay into the different hyperfine levels and identify repumping schemes. We demonstrate our scheme by cycling photons in a molecular beam and verify that we have achieved rotationally closed cycling by measuring optical pumping into unaddressed vibrational states. Our work makes progress along the path toward utilizing photon cycling for state preparation, readout, and laser cooling in precision measurements of polyatomic molecules with complex hyperfine structure.
Rapid and repeated photon cycling has enabled precision metrology and the development of quantum information systems using atoms and simple molecules. Extending optical cycling to structurally complex molecules would provide new capabilities in these areas, as well as in ultracold chemistry. Increased molecular complexity, however, makes realizing closed optical transitions more difficult. Building on already established strong optical cycling of diatomic, linear triatomic, and symmetric top molecules, recent work has pointed the way to cycling of larger molecules, including phenoxides. The paradigm for these systems is an optical cycling center bonded to a molecular ligand. Theory has suggested that cycling may be extended to even larger ligands, like naphthalene, pyrene, and coronene. Herein, we study optical excitation and fluorescent vibrational branching of CaO-[Formula: see text], SrO-[Formula: see text], and CaO-[Formula: see text] and find only weak decay to excited vibrational states, indicating a promising path to full quantum control and laser cooling of large arene-based molecules.
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We demonstrate optical cycling and transverse laser cooling of a beam of fermionic BaF 137 molecules. Their high masses and nuclear spins make these molecules sensitive probes for parity violation and properties of the weak interaction. However, the nuclear spins also lead to a quasiclosed cycling transition currently involving up to 112 levels, which significantly exceeds the complexity in other laser-cooled molecules. Optical cycling and cooling are facilitated through carefully designed optical spectra tailored to this molecular structure. Our results pave the way for efficient state preparation, detection, and cooling in precision measurements using this species and other similar species.
We study optical cycling in the polar free radical calcium monohydroxide (CaOH) and establish an experimental path towards scattering $\ensuremath{\sim}{10}^{4}$ photons. We report rovibronic branching ratio measurements with precision at the $\ensuremath{\sim}{10}^{\ensuremath{-}4}$ level and observe weak symmetry-forbidden decays to bending modes with nonzero vibrational angular momentum. Calculations are in excellent agreement with these measurements and predict additional decay pathways. Additionally, we perform high-resolution spectroscopy of the $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{X}{\phantom{\rule{1.0pt}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}^{+}({12}^{0}0)$ and $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{X}{\phantom{\rule{1.0pt}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}^{+}({12}^{2}0)$ hybrid vibrational states of CaOH. These advances establish a path towards radiative slowing, three-dimensional magneto-optical trapping, and sub-Doppler cooling of CaOH.
Achieving laser cooling of molecules necessitates the establishment of a closed optical cycling transition. In contrast to atoms, molecules exhibit additional vibrational and rotational structures, which must be carefully addressed-alongside hyperfine splittings-when designing the laser frequency scheme for efficient optical cycling. In this work, we investigated optical cycling in MgF molecules on the laser cooling transition, namely [Formula: see text] transition between the electronic ground state ([Formula: see text]) and the electronic excited state ([Formula: see text]). The ground state comprises four hyperfine levels, of which two are unresolved within the excited-state linewidth; the two hyperfine levels in the excited state are also unresolved. Consequently, three laser frequencies suffice to address all transitions. This was realized by generating three independently tunable frequency components using acousto-optic modulators (AOMs). With optimized detunings, power ratios, and total beam power, simultaneous application of all three frequency components increased the total number of scattered photons by up to [Formula: see text] relative to the sum of the individual single-frequency signals. Applying a tilted magnetic field to remix the dark Zeeman states increased the photon yield by an additional factor of [Formula: see text], leading to an overall enhancement of about sevenfold. These results provide quantitative benchmarks for MgF optical cycling and practical guidance for future molecular slowing and magneto-optical trapping experiments.
The bichromatic optical force (BCF), which can greatly exceed radiative forces, seems ideal for laser slowing and cooling of molecules because it minimizes the effects of radiative decay. However, it relies on sustained coherences between optically coupled states, and molecules, with their many sublevels and decay pathways, present new challenges in maintaining these coherences compared with simple atoms. We have conducted extensive numerical simulations of BCFs in model molecular systems based on the BleftrightarrowXB leftrightarrow XBleftrightarrowX transition in CaF, and have begun experimental tests in a molecular beam.\ In our modeling, the effects of fine and hyperfine structure are examined using a simplified level scheme that is still sufficiently complete to include the major pathways leading to loss or decoherence. To circumvent optical pumping into coherent dark states we explore two possible schemes: (1) a skewed dc magnetic field, and (2) rapid optical polarization switching. The effects of repumping to compensate for out-of-system radiative decay are also examined. Our results verify that the BCF is a promising method for creating large forces in molecular beams while minimizing out-of-system radiative losses, and provide detailed guidance for experimental designs. Compared to a two-level atom, the peak force is reduced by about an order of magnitude, but there is little reduction in the velocity range over which the force is effective. Our experiments on deflection and slowing using the CaF BleftrightarrowXB leftrightarrow XBleftrightarrowX, (0-0) transition, still at an early stage, include studies of both the P11(1.5)/PQ12(0.5)P_{11}(1.5)/^PQ_{12}(0.5)P11​(1.5)/PQ12​(0.5) branch, a quasi-cycling configuration with extensive hfs, and the R11(0.5)/RQ21(0.5)R_{11}(0.5)/^RQ_{21}(0.5)R11​(0.5)/RQ21​(0.5) branch, which has a much simpler hfs but requires rotational repumping.